1. Field of the Invention
The present invention pertains to a process for making and to compounds comprising homopolymers, copolymers, and terpolymers (hereinafter referred to as "polymers") of 4-hydroxystyrene and/or substituted 4-hydroxystyrene. The polymers have low optical density (absorbance) over a wavelength range from about 240 to 260 nm, as well as low optical density over the visible and near UV spectrum (310 to 800 nm). Such polymers can be produced using a single compound which serves as a reaction medium for the polymerization, as a chain transfer agent and as a transesterification reactant, providing an unusually economical process. The polymers of the present invention are particularly useful (directly or in modified form) as photoresist materials for deep UV, X-ray, and E-Beam imaging systems.
2. Description of the Background Art
There are numerous applications within the semiconductor industry which require photoresists materials having low absorbance in the deep U.V. (240 to 260 nm) wavelength region. The novolak photoresist polymers which have been used for optical lithography are limited to wavelengths longer than 300 nm because of the high absorbance of the novolak material at shorter wavelengths. It has recently been learned that polymers and copolymers of 4-hydroxystyrene and substituted 4-hydroxystyrenes are particularly useful over a wavelength range from about 240 to 260 nm. The molar absorbtivity (.epsilon.) of such polymers and copolymers over this wavelength range is as low as about 50 l cm.sup.-1 mole.sup.-1.
There is a need for an economic method or process for producing polymers and copolymers of 4-hydroxystyrene and substituted 4-hydroxystyrene.
U.S. Pat. No. 4,822,862, issued Apr. 18, 1989, to R. W. Rupp et al., describes the homopolymerization and copolymerization of 4-acetoxystyrene monomer (4-ASM) in an aqueous emulsion and, without isolation, the hydrolysis of the polymers and copolymers of 4-ASM to homopolymers and copolymers of 4-hydroxystyrene using a base.
U.S. Pat. No. 4,912,173, issued Mar. 27, 1990, to D. L. Keene et al., discloses a process for hydrolyzing homopolymers of 4-ASM to homopolymers of 4-hydroxystyrene. The hydrolysis is carried out in an aqueous suspension in the presence of a nitrogen-comprising base.
European Patent No. 343,986, issued Nov. 29, 1989, to R. Vicari et al., describes the preparation of polymers of 4-ASM or polymers of 4-hydroxystyrene by suspension polymerization. The suspension polymerization comprises forming an aqueous suspension of 4-ASM in the presence of polyacrylic acid and at least 2 free radical catalysts, one of which has a half life of 1 hour below 100.degree. C. and the other which has a half life of 1 hour above 100.degree. C.
The suspension is heated to 70.degree.-95.degree. C. to achieve about 50 wt % polymerization, and then polymerization is completed by heating above 95.degree. C. To form the polymer of 4-hydroxystyrene (4-HSM), the reaction temperature is then reduced to between 30.degree.-95.degree. C. and the suspension is reacted with ammonium hydroxide in an amount of at least 2 moles of ammonia for each equivalent of 4-ASM.
The poly(4-hydroxystyrene) produced by the above-described method, typically has an absorbance at 248 nm which ranges from about 300 to 350 liters/cm.sup.-1 mole.sup.-1 (molar absorbtivity, .epsilon.).
U.S. Pat. No. 4,898,916, issued Feb. 6, 1990, to B. Gupta et al. describes a process for the conversion of poly(4-acetoxystyrene) (4-PAS) to poly(4-hydroxystyrene) (4-PHS). The process comprises: a) forming an alcoholic slurry of (4-PAS); b) adding an acid to the slurry; and c) maintaining slurry temperature at 20.degree.-65.degree. C. until the polymer dissolves in the alcohol, indicating complete acetoxy to phenolic group conversion.
Processes wherein the 4-ASM is polymerized into a homopolymer or a copolymer with another copolymerisable monomer, and especially wherein the comonomer is styrene, are claimed. The 4-PHS or copolymers of 4-HSM is expected to have a molar absorptivity, .epsilon., ranging from about 300 to 1,000 depending on the 4-PAS used as the precursor for the reaction described.
U.S. Pat. No. 4,689,379, issued Aug. 25, 1987, to J .D. Elmore et al., discloses the hydrolysis of polymers of 4-ASM by methanolsis with quaternary ammonium hydroxides to 4-PHS.
European Patent Application No. 87307912.3, published Mar. 16, 1988, discloses a process for preparing polymers comprising 4-PHS by initially reacting 4-ASM and methanol, with subsequent reaction of 4-ASM, methanol, 4-HSM, polymers of 4-ASM, and polymers of 4-HSM, in the presence of an ammonium base using a free radical initiator. The initiator is selected from a group of initiators including benzoyl peroxide, lauroyl peroxide, acetyl peroxide, cumene hydroperoxide, paramethane hydroperoxide, etc.
G. Pawlowski et al. disclosed the polymerization of 3-methyl-4-acetoxystyrene using 2,2'-azobis(2-methylpropanenitrile) in tetrahydrofuran (THF) to produce poly(3-methyl-4-acetoxystyrene) at the March 1990 SPIE meeting held at San Jose, Calif.
The poly(3-methyl-4-acetoxystyrene) polymerized by G. Pawlowski et al. in THF was isolated (precipitated) and dried prior to subsequent use. This dried poly(3-methyl-4-acetoxystyrene) can then be slurried in methanol and processed as described above by B. Gupta et al. to provide homopolymers and copolymers of poly(3-methyl-4hydroxystyrene).
However, preparation of 4-HSM polymers using a combination of the above techniques would be very expensive. The THF solvent used by Powlowski et al. in polymerizing the 4-PAS is toxic. In addition, the precipitation and drying of the 4-PAS adds additional cost in the form of unit operations and waste disposal. In addition, the method of Gupta et al. disclosed in U.S. Pat. No. 4,898,916 is difficult ot accomplish on a large scale due to problems encountered in a making of a slurry of the 4-PAS in alcohol. The 4-PAS tends to "clump" or remain in agglomerated form in the alcohol, due to insolubility.